It is clear that the Myb protooncogene plays a crucial role during hematopoiesis. In each hematopoietic lineage, c-Myb is abundantly expressed at the immature stages of differentiation and is turned off at a relatively late time during the differentiation process. However, virtually nothing is known about what role c- Myb plays during hematopoietic maturation, how that role is mediated or what signaling pathways regulate c- Myb expression. Mice that are homozygous null at the Myb locus die at day fifteen during embryogenesis from a severe anemia. This finding graphically demonstrated the significance of c-Myb during hematopoiesis but has precluded study of c-Myb activity at the later stages of differentiation. The lack of a tractable genetic system that will allow mutation of Myb during the later stages of hematopoietic maturation has been a major impediment to understanding the role of c-Myb during hematopoiesis. During T cell development in the thymus, CD4 and CD8 double negative (DM) and double positive (DP) thymocytes contain abundant amounts of c-Myb while CD4+ and CD8+ single positive (SP) cells contain only 10 to 20% as much c-Myb. In the periphery, c-Myb is expressed at only low levels in resting T cells, but expression increases in proliferating T cells in late G1/early SD phase of the cell cycle. To begin to understand the role played by c- Myb during T cell development we have produced mice that carry a Myb allele targeted with loxP sites for deletion by the Cre recombinase. By breeding these mice to available mouse strains that direct Cre expression to different stages of T cell development in the thymus we have defined critical points during T cell development where c-Myb is required: transition from the DN to the DP stage of development, for the survival of pre-selection DP cells and the differentiation of CD4 SP thymocytes. More recently we have identified roles for c-Myb in maintaining T cell homeostasis and mediating T cell helper function. The goals of this proposal are to understand the basis for c-Myb function during the DN stage of T cell development, in peripheral T homeostasis and helper T cell function.